1. Field of the Invention
The present invention generally relates to oilfield telemetry systems, and more specifically relates to a method for compensating for offset between a downhole clock and a clock in a surface installation.
2. Description of Related Art
Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as xe2x80x9cloggingxe2x80x9d, can be performed by several methods.
In conventional oil well wireline logging, a probe or xe2x80x9csondexe2x80x9d housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
Since the sonde is in direct electrical contact with the surface installation, the communications delay is negligible. Accordingly, measurements can be made and communicated in xe2x80x9creal timexe2x80x9d. If it should be deemed necessary, a downhole clock in the sonde can be easily synchronized with a surface clock. A computer on the surface can reset a surface clock while simultaneously transmitting a reset command to the downhole clock. Any offset due to the communications delay is for all practical purposes insignificant.
The problem with obtaining downhole measurements via wireline is that the drilling assembly must be removed or xe2x80x9ctrippedxe2x80x9d from the drilled borehole before the desired borehole information can be obtained. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. In this situation, thousands of feet of tubing may need to be removed and stacked on the platform (if offshore). Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool can be an expensive proposition.
As a result, there has been an increased emphasis on the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as xe2x80x9cmeasurement-while-drillingxe2x80x9d techniques, or xe2x80x9cMWDxe2x80x9d. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as xe2x80x9clogging while drillingxe2x80x9d techniques, or xe2x80x9cLWDxe2x80x9d. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
A number of techniques have been used to transmit data obtained from LWD measurements to the surface. These include mud pulse telemetry, electronic telemetry, acoustic telemetry, and the like, with the system chosen to accommodate the particular conditions and measurements under consideration. For example, it is impractical to run an electrical cable downhole during drilling operations. Consequently, measured data are communicated by other means such as mud pulse telemetry. In mud pulse telemetry, the flow of drilling mud through the drillstring is modulated by periodically obstructing the flow. The resulting pressure waves propagate upstream and can be sensed at the surface. As another example, when it is desired to detect formation boundaries and to map the structure of earth formations, it is useful to conduct seismic profiling. In seismic profiling, measurements are obtained using sound waves, also called acoustic waves or seismic waves. It is well known that mechanical disturbances can be used to cause acoustic waves in earth formations and that the properties of these waves can be measured to obtain important information about the formations through which the waves have propagated. As it is known in art, the arrival time of these waves via the formation provides very useful information regarding the type of formation.
In these examples and in other LWD systems, clocks are often employed to provide timing information at more than one location. If different locations are subjected to varying conditions, such as temperature and pressure, this may result in clock desynchronization. For LWD techniques where it is desirable to compensate for clock error, this issue has not been adequately addressed. Seismic profiling is one such technique.
In a basic version of seismic reflection profiling, an acoustic source is used to send a sound signal from the earth""s surface, at an initial time. The signal travels down through the earth, reflecting off boundaries between different formation features. A portion of the reflected signal travels back to a receiver, which registers the intensity of the signal as a function of the time elapsed from the initial time. This allows the time to travel to and from a formation feature to be measured. If the speed of the signal is known, then the travel time can be converted to the distance from the surface, or depth, of the feature. Time measurements are typically made with reference to one or more clocks.
Variations on this basic method of seismic reflection profiling are known. In particular, in vertical seismic profiling, a plurality of seismic receivers are placed in the borehole, with each receiver being at a different depth in the borehole. These receivers are used in conjunction with seismic sources placed either on the surface or inside another well. In reverse seismic profiling a downhole source is used. The source may be the drill bit itself or an alternate source placed downhole. The downhole source is used in conjunction with a plurality of seismic receivers placed at different points on the surface. Combinations of these techniques of vertical seismic profiling and reverse seismic profiling are also known, including three and four dimensional seismic profiling.
The speed of the acoustic signal used in seismic profiling varies with the material through which the signal travels. Therefore calibration of the seismic profiling measurements must be performed by measuring the acoustic travel time for a known distance. A measurement signal is known as a shot. A calibration signal which is used to obtain the speed of the sound wave is known as a checkshot. Calibration is performed by sending a signal a known distance and measuring the travel time. The time measurement is made with reference to one or more clocks.
Wireline checkshots may be used in conjunction with LWD shot measurements. In a traditional wireline checkshot a clock is associated with a seismic source on the surface. Another clock is associated with a receiver at the end of cable, which is lowered a known depth into the borehole. The industry has common downhole position measurement techniques that are known and may be used with checkshot measurements. The source clock is used to record the initial time of generation of the signal. The receiver clock is used to record to the time at which the signal reaches the receiver. Subsequent LWD shots can then be calibrated from the wireline checkshot.
As mentioned previously, wireline methods have the disadvantage that the drillstring must be tripped, causing delay and expense. Alternatively, tripping the drillstring may be avoided, by performing the checkshot while drilling, although drilling may be temporarily halted to reduce noise. An LWD checkshot may be carried out, for example, by transmitting the calibration signal along the casing or along the drillstring.
In LWD, the downhole clock may remain in the borehole for days at a time. Because the clock is sensitive to temperature, it will drift relative to the surface clock and lose synchronization. The magnitude of the drift on a crystal-based clock may be on the order of 30 milliseconds a day. When an accuracy of milliseconds is desired, this drift is unacceptable. An accuracy of milliseconds, for a representative speed of sound of 7.54 ft/msec, is equivalent to a spatial accuracy of 5-10 ft. This level of accuracy is necessary for evaluating reflections from thin, stratified formations, which may be capable of efficiently producing hydrocarbons. Therefore it is desirable to have a method to correct for the clock drift and compensate a downhole clock with a surface clock with millisecond accuracy. One method of compensation is to synchronize the clocks, that is to shift their time origins to be the same.
U.S. Pat. No. 5,850,369 describes a system in which acoustic transceivers are time synchronized. Initial synchronization is accomplished through transmission of a synchronization signal in the form of a repetitive chirp sequence by one of the units, such as the downhole acoustic transceiver. A second synchronization signal is transmitted from the surface acoustic transceiver (SAT). The second synchronization signal is comprised of two tones, each of a different frequency. Signal analysis of these tones by the downhole acoustic transceiver (DAT) enables the timing of the DAT to be adjusted in synchrony with the SAT.
There remains a need in the art for an accurate, precise, uncomplicated, and self-contained method to correct for desynchronization between a downhole clock and a surface clock for use in LWD systems that rely on acoustic telemetry.
Accordingly, there is provided herein a system to determine accurately the clock offset between a downhole clock and a surface clock. A pair of acoustic signals is exchanged between downhole and surface locations. Each clock is reset at a particular time with respect to one of the acoustic signals. The clock offset is obtained by processing each acoustic signal to accurately determine a travel time for each acoustic signal.
In one embodiment, the system includes a downhole acoustic apparatus for transmitting, receiving, and processing acoustic signals placed in a downhole sensor sub. The downhole acoustic apparatus is coupled to a downhole clock. The downhole acoustic apparatus includes a sensor coupled to a receiver for receiving a reset acoustic signal transmitted from the surface and a signal processor coupled to the receiver for determining the time of arrival of the reset acoustic signal and causing the clock to be reset. The signal processor may further include an analog/digital converter (ADC). The sampling rate of the ADC is several times higher than the frequency of the acoustic reset signal, preferably at least 10 times faster. The frequency of the signal processor is preferably several times higher than the sampling rate of the ADC. The downhole acoustic apparatus further includes a transmitter coupled to the signal processor and to a transducer for transmitting a return acoustic signal to the surface.
The system also includes a surface acoustic apparatus for transmitting, receiving, and processing acoustic signals, which is placed near the top of the drill string. The surface acoustic apparatus is coupled to a surface clock. The surface acoustic apparatus includes a transmitter coupled to a transducer for transmitting the reset acoustic signal and to a signal processor for causing the clock to be reset as the reset signal is transmitted. The surface acoustic apparatus further includes a receiver coupled to the signal processor and to a sensor for receiving the return acoustic signal.
Each signal processor is configured to determine the time of arrival of an acoustic signal by analyzing some characteristic of the acoustic signal. Examples of suitable characteristics include the root mean square average, the phase of the Hilbert transform, and a difference between the original signal and a multiple of a delayed version of the signal. The surface signal processor is further configured to determine a clock offset from the delay between transmission of the reset acoustic signal and receipt of the return acoustic signal.
An alternative embodiment of the system may include an intermediate acoustic apparatus for relaying acoustic signals between a downhole acoustic apparatus and a surface acoustic apparatus. The use of an intermediate acoustic apparatus may advantageously improve the reliability of the procedure to determine a clock offset between a downhole and a surface clock when the downhole clock is below the mud motor.
In another alternative embodiment the downhole acoustic apparatus is capable of receiving a sequence of acoustic signals following the reset acoustic signal. The downhole signal processor is capable of determining the arrival times of the subsequent signals. Further the signal processor can advantageously process the arrival times to ascertain and store the surface clock information for time stamping subsequently measured LWD data.